Method of and means for controlling corona emission

ABSTRACT

A method of controlling the area developed by a corona discharge which consists in providing a shield around the corona point and controlling the effect to the shield to have the focussing effect on the corona preferably by controlling the position of the electrical charge or the electrical resistivity of the shield.

United States Patent [191 Wright [5 METHOD OF AND MEANS FOR CONTROLLINGCORONA EMISSION [75] Inventor: Robert J. Wright, Tranmere, Australia[73] Assignee: Research Laboratories of Australia Pty. Limited,Eastwood, South Australia, Australia 221 Filed: NOV.2, 1970 21 Appl.No.:85,964

[30] Foreign Application Priority Data Nov. 7, 1969 Australia ..63498 521 7 US. Cl. .:.250/49.5 GC, ZSOI EE ZC:

250/495 TC 511 Int. Cl ..I*l0lj 37/26 [58 Field of sarcn....."250 49.soc, 49.5 zc, 49.5 a

[ 1 Jan. 16, 1973 Primary Examiner-Archie R. Borchelt Att0rneyl(inzer,Dorn and Zickert [57] ABSTRACT A method of controlling the areadeveloped by a corona discharge which consists in providing a shield'around the corona point and controlling the effect to the shield tohave the focussing effect on the corona preferably by controlling theposition of the electrical charge or the electrical resistivity of theshield.

4 Claims, 3 Drawing Figures ILL/I III/l H l6 H VOLTAGE SUPPLY I III/I771PATENTEDJAH 16 I973 H lG H VOLTAGE SUPPLY 9 (PHoTocorwucToFz) METHOD OFAND MEANS FOR CONTROLLING CORONA EMISSION This invention relates to thereproduction of visual information, and is particularly applicable tosignal recording and character printing processes in which an inputsignal is used to control in patterned form the impression ofelectrostatic charge on the dielectric surface of an electrographicrecording member.

Various methods of recording of information on electrographic surfacesare known and such methods generally provide for the production of apattern of electrostatic charge on the surface of the electrographicrecording member. The electrographic recording member may in itssimplest form comprise a paper or other relatively conducting backingmember having coated on one surface thereof a thin insulator film, suchas a polyester resin or the like. The charge pattern may be controlledby an input signal which controls the selection of those areas on theelectrographic surface which are to be charged, and prevents charging ofthose areas of the electrographic surface which are required to remainuncharged. Development of the soproduced charge patterns by the use ofeither dry or liquid dispersed toner materials, such as those well knownin the art of electrophotography, results in the production of a visualrecord of the information impressed as an electrostatic latent image onthe electrographic recording member surface. Single point or multipointcharging means may be used, the input means providing for controlleddeflection of the single point, or selection of individual points in themultipoint system. The single point system may involve the use of astylus which is moved across the surface as is known for instance inchart recorders and the like, whereas the multipoint system isillustrated by such character printing devices as those employing amatrix of charging points, wherein the points are arranged in a closedrectangular pattern containing finite parallel rows of points which maybe selectively energized to form a charge pattern corresponding to aletter of the alphabet or a number or some other desired pattern ordesign. Alternatively the matrix of charging points or styli may form alinear array as is common in bit recording devices.

A disadvantage of the prior art systems relates to their need to havethe charging points very close to the surface being charged, and in factin many instances it is necessary to position the charging points sothat they are actually in contact with the electrographic surface. Thishas been necessary in prior art processes in order that charge spreadingon the recording surface may be held to within acceptable limits.Contact between the points and recording surface is a disadvantage forthe following reasons. Firstly, this causes wear on the points, and thuschanges their geometry, which in turn affects the shape of the chargeareas produced therefrom. Secondly, the contact of charging points withthe electrographic surface may give rise to spurious signals,particularly in those instances in which the electrographic recordingmember is moved during image production. Thirdly, equipments utilizingcontact or near contact charging methods need to be manufactured toconform with very small tolerances in order that they may produceaccurate and uniform records of input signals.

This present invention teaches a method whereby the disadvantages ofprior art processes may be overcome, in that it provides a method forrestricting the zone of influence of a charging point, whereby the sizeof an area charged by a point electrode may be restricted to 0.010 inchdiameter or less, even when the point is several inches removed from thesurface being charged. Thus the present invention teaches a method forcharging dielectric surfaces in patterned form which consists of placingthe member to be charged on a grounded backing member with itsdielectric surface spaced apart therefrom, characterized by such coronacharging point being substantially surrounded by a planar shieldadjustably positioned in relation to said corona charging point wherebythe zone of influence of said corona discharge point on said dielectricsurface may be varied in size by varying the position of said shield. Inaddition the size of the area of such zone influence may also be alteredby varying the electrical resistivity of said shield, or by varying thecontact resistivity between said corona charging point and said shield.

The diameter of the shield appears to be relatively non criticalprovided it is substantially in excess of the diameter of the stylusshank so that secondary emission from the shield is of insufficientstrength to deposit unwanted charge on the recording surface. It will beobvious to those skilled in the art that the effect of such secondaryemission may be minimized particularly where the diameter of such shieldis kept to a minimum by rounding the edges thereof or by coating suchedges with an insulating material.

it is known that a single point corona charging electrode, spaced apartfrom a grounded backing member at a sufficient distance to prevent theproduction of a hot spark when a high voltage power supply connected tosuch corona point is energized, will produce a relatively soft edgedelectrostatic charge on the surface of an electrographic orelectrophotographic sheet placed on the grounded backing member andbetween such backing member and the corona charging point electrode, thediameter of such charged area being somewhat larger than the distancebetween the point and the grounded backing member when anegative highvoltage is connected to such corona point, while the charged areaproduced by positive corona is usually somewhat smaller than thatproduced by a negative corona. If the spacing between the point and thegrounded backing member is increased, the size of the charged area isalso increased without improving the edge delineation of such chargedarea.

In accordance with the present invention a further member is provided inthe charging system, such member being of selected electricalconductivity, and in addition being in a preferred embodimentsubstantially planar, and having a hole in the center thereof throughwhich the tip of the corona point protrudes. The shank of the coronapoint electrode may touch the planar shield member in some instances,whereas in other instances there may be no direct physical contact andthus no direct electrical contact between the corona point electrode andthe planar shield member. The effectiveness of the corona restrictionproduced by the introduction of such a shield member is influenced bythe geometry of the system and the electrical resistivity of theshieldmember. Geometrical features which influence the effectiveness of coronarestriction include the distance the corona point projects through theshield, the shape of the point, and the distance between the point andthe grounded backing member, while further control of the size of thecorona projected area may be obtained by varying the electricalconductivity of the shield member, as well as by varying the voltageapplied to the corona point.

In order that the invention may be more readily understood referencewill now be made to the illustrations, in which:

. FIG. 1 illustrates theembodiment in which the shield member isadjustably mounted in relation to the charging point,

FIG. 2 illustrates how the corona emission may be modulated, and 1 FIG.3 shows how modulation can be carried out by a light using aphoto-conductor shield.

It will be realized that these illustrations are intended to depict theprinciples of the invention only and are not included in the restrictivesense. In FIGS. 2 and 3 the high tension supply will be similar to thatshown in FIG. 1. Corresponding parts in each figure are indicated bysimilar reference numerals.

Referring now to FIG. 1 in detail, a grounded conductive backing member1 has placed thereon an electrographic recording member consisting of arelatively conductive backing 2 in contact with said grounded base 1with a dielectric surface layer 3 on the side of said backing 2remote'from the gounded base 1. A corona charging point 4, connected toa high voltage power supply, is positioned to face the dielectricsurface 3, and spaced apart from the dielectric surface. A conductiveshield member 6, substantially planar on the side facing the dielectricsurface 3, is adjustably mounted to the corona charging point 4, theshield member 6 may be moved up and down in response to an electricalsignal to vary the size of the area on the dielectric surface 3 which ischarged when high tension is applied to the corona charging point 4.

FIG. 2 shows an alternate shield construction in which the conductiveshield'6 is separated from corona charging point 4 by insulator 7, and avariable resistor 8 is connected between the charging point 4 and theshield member 6 to provide the modulating control.

FIG. 3 shows a further alternate shield construction in which shield 6is replaced with a photoconductor shield 9, the resistivity of which ismodulated by varying the intensity of lamp 10. The power supply isnumbered 1 1.

It will be realized that the shields of each of FIGS. 2 and 3 may be inaddition moved in relation to the corona charging point 4 to obtainfurther modulation of the size of the area charged on dielectric surface3.

The following examples will serve further to illustrate the invention.

EXAMPLE 1 A corona point charging device was constructed in which apoint, radius 0.002 inch, was formed on the tapered end of a steel shaft0.060 inch diameter, the included angle of the tapered section of theshaft being The shaft was connected to the output terminal of a highvoltage power supply, arranged to supply a negastatic latent image wasdeveloped by immersion in a,

bath of liquid developer of the type used in electrophotographicdocument copying machines and the like.

Application of 20KV negative to the point, with the point to backingplate distance set at 1.25 inches resulted in the production of a softedged corona discharge pattern approximately 2.25 inches diameter beingproduced on the sheet of electrographic paper placed on the backingmember. In this instance the shield member was not used, and the edgesof the charge pattern were ill defined.

A shield member was then prepared, which member consisted of a bronzedisc, 1.19 inches diameter, 0.059 inch thick, substantially planar, witha central hole 0.060 inch diameter. The electrode point was pushedthrough the hole in the shield member and protruded 0.19 inch below theshield member. The corona pattern produced by the application of apotential of 20KV negative to the point, with the point to backing platedistance set at 1.25 inches, was 2.125 inches diameter, with a sharplydefined edge.

The shield was then moved slightly tochange the distance which the pointprotruded through the shield to 0.125 inch. The sharply defined coronapattern so produced was 1.25 inches diameter.

A further series of tests carried out with the same charging point andshield member resulted in the production of a sharply defined coronapattern 0.031 inch diameter when the point to backing member distancewas set at 0.625 inch and the point protruded 0.0 l 6 inch through theshield member.

EXAMPLE 2 4 The following series illustrates variations in coronarestriction brought about by changing the resistivity of the shieldmember. In each instance the applied voltage was 20 KV negative, withthe charging point protruding 0.25 inch through the shield, and a shieldto backing member distance of 1.50 inches. l

A polyethylene shield produced a sharp fedged corona pattern of 4.0inches diameter.'The volume resistivity of the of the polyethrylene was1.6 X -10 ohm.cm. I

A cardboard shield produced a sharp edged corona pattern of 1.75 inchesdiameter. The volume resistivity of the cardboard was about 2.5 X 10ohm.cm.

A bronze shield produced a sharp edged corona pattern of 1.24 inchesdiameter.

EXAMPLES 5 6 A cardboard shield, volume resistivity 2.5 X 10 ohm.cm with20KV applied to a point protruding 0.5 inch through the shield, andspaced 1.50 inches above the backing member produced a charge pattern1.50

inches diameter when a negative polarity was applied to the point,whereas the charge pattern was 1.25 inches diameter when a positivepolarity was applied to the point.

It will be seen from the foregoing that the restriction of the zone ofinfluence of a corona beam may be controlled by the provision of ashield member, and that when all other factors are kept constant, theextent of such restriction is dependent on the resistivity of the shieldmember, the distance the charging point protrudes through such shieldmember, and the distance between the charging point and the backingmember.

It will be realized that the examples are illustrative of the principleof the invention only, but are indicative of the control methodsavailable for modulating corona beams, which modulation may be achievedby moving the shield in relation to the point, varying the volumeresistivity of the shield or a combination of some or all of thesemeans. In addition the recording member may be moved to produce acontinuous trace where this is desired. In addition the high voltagepower supply may provide a steady DC output, or may provide an interrupted DC output of a selected frequency, or of a modulated frequency orthe amplitude or frequency may be modulated, whereby the trace obtainedon the recording member may be discontinued or attenuated where this isrequired. Thus a record of an external signal may be produced by causingsuch signal to provide mechanical modulation in accordance with therequirement of any or all of the variables herein disclosed.

As previously stated, the electrostatic latent image may be developed bythe use of a dry electrophotographic toner material, or preferably bythe use of a liquid dispersed electrophotographic toner. A liquidelectrophotographic toner in dispersion in an insulating carrier liquidproduces anelectrophotographic liquid developer, which may be defined asan electroscopic marking material suspended in a carrier liquid, whereinthe carrier liquid has a volume resistivity greater than ohm. cm anddielectric constant less than 3, and wherein such electroscopic markingmaterial comprises a pigment or dye or other colored particle combinedwith a resinous or oleoresinous or other fixing or dispersing agent, andin addition combined if necessary with a polarity control agent such asan alkyd resin or other material well known in the prior art, suchcombined particle forming the toner which is in suspension in theinsulating carrier liquid. Such dry or liquid dispersed toner materialsmay be formulated and compounded to be attracted to either a positive ora negative electrostatic charge as required.

While in the foregoing reference was made to electrographic paper as thecharge receptive film or recording member, it will be realized that suchcharge receptive film may comprise a photoinsulator and thus therecording member may be electrophotographic paper or otherphotoconductive printing element if desired without departing from thespirit of the herein described invention.

As a further alternative the dielectric recording member need not have aconductive backing.

1 claim: 1. A method of charging a dieelectric surface of a recordingmember in patterned form comprising the following steps:

A. mounting a point-discharge corona discharge member in close proximityto a grounded backing member with the charging point of said dischargemember directed toward but spaced from said backing member;

B. positioning a shield member in encompassing relation to said chargingpoint of said discharge member, said shield member having a relativelylarge, continuous planar surface facing said backing member within arange in which the charging point of the discharge member extends beyondthe plane of the shield surface by no more than a fraction of an inch;

C. positioning a recording member on said backing member with a givenportion of the dielectric surface of said recording member facing saidcharging point;

D. energizing said corona discharge member from a high voltage powersupply to generate a corona discharge between said charging point ofsaid discharge member and said recording member;

E. adjusting the position of said shield member, relative to saidcharging point, within a range in which the charging point of thedischarge member extends beyond the plane of the shield surface by nomore than a fraction of an inch, to produce a charge area on saiddielectric surface that is substantially smaller than and more sharplydefined than the charge area produced by said point, at the sameposition and with the same energization, in the absence of said shieldmember;

. and repeating steps C and D, for different portions of the dielectricsurface of said recording member, to produce a pattern of sharplydefined charge areas on said recording member surface.

2. A method for charging dielectric surfaces in accordance with claim 1,further characterized in that control of the size of the area charged isobtained by the step of varying the electrical resistivity of saidshield.

3. A method for charging dielectric surfaces in accordance with claim 1,further characterized in that control of the size of the area charged isobtained by the step of varying the resistance between the coronacharging point and the shield.

4. A method for charging dielectric surfaces in accordance with claim 1further characterized in that the charge on the said shield iscontrolled by actuating a variable resistance means between the coronacharging point and the shield.

1. A method of charging a dieelectric surface of a recording member inpatterned form comprising the following steps: A. mounting apoint-discharge corona discharge member in close proximity to a groundedbacking member with the charging point of said discharge member directedtoward but spaced from said backing member; B. positioning a shieldmember in encompassing relation to said charging point of said dischargemember, said shield member having a relatively large, continuous planarsurface facing said backing member within a range in which the chargingpoint of the discharge member extends beyond the plane of the shieldsurface by no more than a fraction of an inch; C. positioning arecording member on said backing member with a given portion of thedielectric surface of said recording member facing said charging point;D. energizing said corona discharge member from a high voltage powersupply to generate a corona discharge between said charging point ofsaid discharge member and said recording member; E. adjusting theposition of said shield member, relative to said charging point, withina range in which the charging point of the discharge member extendsbeyond the plane of the shield surface by no more than a fraction of aninch, to produce a charge area on said dielectric surface that issubstantially smaller than and more sharply defined than the charge areaproduced by said point, at the same position and with the sameenergization, in the absence of said shield member; F. and repeatingsteps C and D, for different portions of the dielectric surface of saidrecording member, to produce a pattern of sharply defined charge areason said recording member surface.
 2. A method for charging dielectricsurfaces in accordance with claim 1, further characterized in thatcontrol of the size of the area charged is obtained by the step ofvarying the electrical resistivity of said shield.
 3. A method forcharging dielectric surfaces in accordance with claim 1, furthercharacterized in that control of the size of the area charged isobtained by the step of varying the resistance between the coronacharging point and the shield.
 4. A method for charging dielectricsurfaces in accordance with claim 1 further characterized in that thecharge on the said shield is controlled by actuating a variableresistance means between the corona charging point and the shield.